CHICAGO — An ambitious, costly plan to expand this city’s police force by almost 1,000 officers to stem a surge in murders and shootings has opened a new question here: Do more police officers always mean an end to rising violence?

On Wednesday, Police Superintendent Eddie Johnson stood before a room crowded with uniformed Chicago police officers and announced plans to increase the department by 970 sworn officers, the largest such expansion here in years. “We can’t rob Peter to pay Paul when it comes to the safety of our city,” Mr. Johnson said. “We need more patrol officers, and we need them where they’re needed the most.”

The expansion would bring the Chicago police force — already the nation’s second-largest, after New York’s — to more than 13,500 sworn officers over the next two years. With more than 500 murders in Chicago already this year and more than 3,000 people shot, the proposed increase is being lauded by many who say that the police here are clearly stretched too thin, that the bloodshed must end, and that the Chicago police need to have time to solve many more cases than they do now.

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Police Superintendent Eddie Johnson speaking to police officers and members of the news media on Wednesday.CreditTaylor Glascock for The New York Times

Yet policing experts and criminologists say that increasing the size of a police force does not ensure a decrease in crime. Jim Bueermann, the president of the Police Foundation, a nonprofit group focused on improving policing, said that once a police department reached a needed minimum number of officers, the equation was not as simple as more police equals less crime. He said that other issues must be weighed, like what roles existing officers were filling and whether adding officers would cause added stress in communities where tensions between the police and residents were already high.

“The traditional way of handling this is to throw a lot of cops at the problem,” Mr. Bueermann said. “But this is not an automatic cure. It depends how you use the officers you have. And you have to be cautious about the unintended consequences of adding police officers. You can over-police. If you get to the point where everyone is getting stopped, everyone is getting towed, nothing good comes from that.”

In the months since a video of Laquan McDonald, a black teenager, being shot 16 times by a white Chicago police officer was made public, the city has been under intense pressure to heal a longstanding sense of distrust and tension between some residents, especially in black neighborhoods, and the police. At the same time, though, the increase in violence has created a different pressure: to end the shootings. Mr. Johnson’s policing plan was announced a day before Mayor Rahm Emanuel is to present a much-anticipated speech on public safety.

Under the plan laid out on Wednesday, Chicago would add police officers at many levels over two years: 516 officers, 92 field-training officers, 200 detectives, 112 sergeants and 50 lieutenants. Mr. Johnson did not address how much it would cost or how the city would pay for all of it in the coming year’s budget, except to say that Mr. Emanuel had given his full backing. Some estimates have put the cost of the expansion at more than $130 million in just the first year, even as Chicago faces a dire fiscal situation, in part because of underfunded pension systems.

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A Chicago police officer investigating a shooting in May. There have been more than 500 murders in Chicago this year and more than 3,000 people shot.CreditJoshua Lott for The New York Times

Chicago has long had a relatively high number of police officers for its population of 2.7 million, compared with other major cities. A Justice Department report showed that the city had 44 full-time sworn police officers per 10,000 residents in 2013, compared with 41 such officers in New York City, which the report said had about 34,400 full-time sworn police officers to cover a population of over eight million and the millions more commuters and tourists who flow into the city.

But some who study policing say setting generic police-per-resident goals is no longer viewed as the most effective way to choose reasonable staffing levels for departments. They said Chicago’s circumstances were such that more police were clearly needed, and that adding supervisors could help change the culture of the department.

“Do more cops by itself reduce crime? No,” said Chuck Wexler, executive director of the Police Executive Research Forum. “But Chicago needs a shot in the arm. Chicago has a unique situation. It has more gangs and guns than any city in the country. I think it needs an infusion.”

Jens Ludwig, the director of the Crime Lab at the University of Chicago, also noted that findings by criminologists that hiring more police did not necessarily reduce crime often had not taken into account a practical side of the question: Cities often hire more police after crime has already gone up. “In the end, I think the best research shows that hiring more police does reduce crime,” Mr. Ludwig said. “And the value of public safety gains can outweigh the investments by the city.”

A version of this article appears in print on , on Page A22 of the New York edition with the headline: Chicago to Hire Many More Police Officers, but Effect on Crime Is Debated. Order Reprints | Today’s Paper | Subscribe